1. Field of the Invention
The present invention relates to an information recording apparatus and, more particularly, to an information recording apparatus capable of compressing information by variable-length coding or the like, as well as capable of varying the compression ratio at which the information is to be compressed.
2. Description of the Related Art
An image information recording or reproducing apparatus, such as a digital video tape recorder (VTR), is typically arranged to compress image information and record the compressed image information on a magnetic tape. In general, compression coding systems are classified into fixed-length coding and variable-length coding, and the variable-length coding has received special attention from the viewpoint of compression efficiency.
More specifically, in the variable-length coding, a code of reduced length can be assigned to data which occurs more frequently, so that the variable-length coding can achieve a higher compression ratio than the fixed-length coding. For this reason, development of compressed-signal recording apparatus using the variable-length coding has recently been performed because of such a high compression efficiency.
As is known, in a VTR using such a variable-length coding, the amount of image information inputted per unit time is always constant, and the amount of information recordable per unit time is likewise constant as long as the operation of the recording apparatus is constant. For this reason, a digital VTR is also proposed which is capable of managing its compression ratio so that the amount of recorded information can be always kept constant per predetermined time unit, i.e., per field (or frame).
The operation performed by this kind of VTR during the variable-length coding is apparently similar to that of a conventional analog VTR or that performed by a digital VTR during the fixed-length coding.
It is to be noted, however, that if the variable-length coding is employed, since it is necessary to perform compression according to a target compression ratio, a coding system having a compression ratio which can be flexibly varied must be adopted.
In addition, since a difference in image quality occurs depending upon the density of an image in the case of the VTR in which the amount of recorded information per field (or frame) is kept constant, a VTR may be considered of the type in which the amount of recorded information per field (or frame) is variable with the compression processing being fixed. In such a VTR, the compression ratio per field (or frame) also diversely varies depending on images. The VTR can, therefore, record an image signal of constantly stable image quality.
However, this kind of VTR encounters the following problem.
The VTR has the disadvantage that the amount of data per field (or frame) is not kept constant, so that it is difficult to achieve an insert-recording in which recorded image information for one or more pictures is replaced with image information indicative of another picture or other pictures. More specifically, no problem may occur if an image having data which is smaller in amount than that of an old recorded image is to be newly recorded in a recording area occupied by the old recorded image. However, if the former amount exceeds the latter amount, the recording of the new image will destruct information recorded in the succeeding recording area.